It has been known since the 1950s that the reversible chemisorption of reactive zones at the surfaces of certain metals, oxides and chalcogenides could be accompanied by reversible changes in conductance. This property has stimulated research and development in the use of semiconductor gas sensors for detecting the presence and amount of certain gases.
Growing concern for worker safety has stimulated development of research in this area for automatic toxic gas detection systems; however, the cost of available sensing systems is too high to justify widespread use. For example, automatic catalytic and electrochemical carbon monoxide detection systems are employed in mining and manufacturing facilities, but are too expensive for home use. There is a growing need for monitoring of carbon monoxide buildup in family residences, particularly new homes that are too airtight and where concentrations of toxic gases, such as carbon monoxide, can build up to dangerous levels. The utilization of semiconductor materials with impedance modulated by interaction with an adsorbed gas as a means of monitoring gas concentrations for carbon monoxide detection has required sensor temperatures maintained in the range of 300.degree.-400.degree. C. (570.degree.-750.degree. F.). This high temperature poses a safety hazard as well as requiring higher fabrication costs and energy use demands. At lower to moderate sensor temperatures, poor sensitivities result. The high sensor temperature requirement has been thought to be needed to achieve reversible carbon monoxide oxidation while, at the same time, maintaining high carbon monoxide sensitivity. Windischmann describes operation of a thin-film, tin oxide, conductance modulation carbon monoxide sensor in, "A Model for the Operation of a Thin-Film SnO.sub.x Conductance-Modulation Carbon Monoxide Sensor," J. Electrochem Soc., Vol. 126, No. 4 (April 1979). Tests were performed with the semiconductor condenser heated to a temperature of from 200.degree.-500.degree. C. Outside of this range, the sensor ceased functioning. Within the temperature range of the sensor, the conductance of the sensor increased with the increase in the partial pressure of carbon monoxide in ambient gas.
The major design criteria for residential detectors for gases include (1) low cost, (2) automatic low maintenance, (3) safety, and (4) good sensitivity under a variety of background gas atmospheres.